Hello and Skifree,
From testing in our latest internal build, the issue seems to already be resolved, as we were able to reproduce this in 4.18 but not on our latest. This means that I will not be placing a bug report in for the issue. The fix should be present in 4.19’s release.
In the meantime, for Windows users, I would suggest launching the editor with -nohighdpi to disable High DPI which is likely causing this issue. For Mac users, the equivalent would be to disable HighResolutionCapable within Info.plist (Engine/Binaries/Mac/UE4Editor.app/Contents).